Getter for electron discharge tubes



Eeb. s, 1944.

R. A. STEPHEN GETTER FOR ELECTRON DISCHARGE TUBES Filec} Sept. 29. 1943 RO/VAL 0 A. STEP/vf/V VINVENTOR.

3 4. Maw 1 Patented F eh. 8, 1944 Ronald Austin Stephen,

signor to The Hartfor- Trust Company, Hartfo Application September 29, 1943, Serial No.

a In Great Britain August 19, 1942 London, England, as- 11 National Bank and rd, Conn., as trustee 8-Claims. (o1. 250-275) In the manufacture of electric discharge tubes various materials have been proposed as getters for improving or maintaining the vacuum in high-vacuum tubes. "Certain of these getters, such as zirconium, tantalum and titanium, operate as getters when heated and accordingly it is known to arrange them within high-vacuum tubes in such position as to be heated to the appropriate temperature during the operation of the tube. This is permissible as they do not contaminate tungsten filaments nor do the resulting compounds which may be formed with the gas give rise to troublesome secondary emission.

According to the present invention aioil of a getter which becomes operative when heatedis, disposed in the path of the discharge and is apertured to permit the passage of the discharge. Advantageously a continuous foil of the heatresponsive getter is mounted in the path of the discharge and during manufacture of the discharge tube is apertured by cathode bombardment. Preferably the foil is first degassed by raising it to the desired temperature by bombardment from the cathode during pumping, the emission of the cathode being adjusted to a suitable low level appropriate to the degassing temperature. Thereafter the bombardment .is increased to evaporate that part of the foil which is in the direct line of the discharge, the evaporated getter being deposited on the adjacent portions of the discharge chamber in the form of a mirror, whereas the apertured foil remains in the original position where it is heated during the operation of the tube and is enabled also to con. tribute to theabsorbtion of gas which may be released in the discharge chamber- Naturally two or more or different materials may be arranged in the pathof the discharge instead of the single foil above referred to. Degassing and partial disintegration by cathode bombardment can be carried out in an equivalent manner. I If desired, the getter foils arranged in the discharge space may be utilised in conjunction with alkaline earth or other getters which have to be kept remote from the discharge space and be in communication therewith only through long winding channels owing to the contaminating effect they exercise on hot tungsten filaments and the large secondary emission which may be produced from the compounds formed with the gas taken up. Such combination of getters is particularly desirable when it is necessary to deal getter foils of the same siderations of efiiciency at comparatively low temperatures and held in a cool place in the tube.

The present invention is particularly applicable to X-ray tubes, where in general, owing to conin X-ray production and/or quality of radiation, the getter materials constituting the foils in the discharge space can- .not be utilised as the anode.

' In order that the invention may be more readily understood, reference will now be had'to the ac companying drawing, which is semi-diagrammatic in character and shows only those parts of existing apparatus which are necessary for describing the present invention.

In the drawing:

Fig. 1 is substantially a longitudinal section through .a glass envelope X-ray tube with -a hooded anode, Fig. '2 is a cross-sectional view in perspective showing the getter assembly'of the 'X-ray tube of- Fi 1.

.Fig. 3 is a fragmentary sectional view'of another X-ray tube in which the central part of the tube envelope is of metal and a metal part1'-, tion is provided in the space between anode and cathode. I

Referring first to Fig. 1, the glass envelope I accommodates'the cathode 2 and the anode 3.- The'heating wires to the cathode are sealed in the envelope in the usual way. The seal for the anode 3 is provided by a chrome-iron envelope portion 4' which is welded or otherwise secured to the anode 3, a fused-joint-being provided betweenglass l and chrome-iron 4. The anode 3 is provided with a hood 5 which surrounds the discharge path adjacent the target surface. Adjoining the target surface, the hood 5 is provided with a window 6 for the unimpeded emergence of the X-ray beam. "The construction as hereinbefore described with reference to Fig. l is of conventional character and accordingly the details are not fully illustrated o described. In accordance withthe present invention, a foil ofheat-responsive getter material, preferably zirconium is interposed in the path of the discharge between cathode 2 and anode 3. The foil 9 is mounted to cover the central aperture 2| of an apertured spider or ring 7 against which it is clamped by a similar ring 8. The rings 1 and 8 are welded or otherwise secured together so vas to hold the foil 9 immovably between them. The rings I and 8 fit within the hood 5 of the anode 3 to should be 7 support the foil in the desired position. Preferably the ring 7 is provided with a flange to give with a gas such as hydrogen which may be freed an enlarged seating surface within the hood 5 2 as shown. As best seen from Fig. 2 both rings 1 and 8 are provided with peripheral openings it] beyond the foil so that the space within the hood between the foil 9 and the target surface is in free communication with the rest of the interior of the envelope I.

The foil 9 may be pre-fabricated with a central opening 20 to permit the passage of the discharge between cathode 2 and anode 3 during normal running of the X-ray tube. Preferably however, a continuous, i. e., non-perforated foil pearance of Fig. 1 except that the foil 9' has a.

central opening which is just the size required fol-"the passage of the discharge as shown at 29 in Fig. 2. In the subsequent use of the tube, the dis,

charge passes close to and heats the remaining foil 9 which consequently exercises its getter action and contributes to the maintenance of the requisite high vacuum.

1 and the In 'the modification of Fig.3, the X-ray tube is of the known type in which a central portion H of the envelope is made of metal and is attached by fused joints to glass end-pieces i2. The central metal portion H carries a partition 55 with a central opening of considerably greater diam-, eter than is requisite for the passage of the discharge. The metal portion l l with the partition I5 is usually connected to earth, whereas the anode l4 and cathode 13 are supplied with equal positive and negative voltages. In the foregoing respects, the construction of Fig. 3 is welltknown. According to the present invention, the partition l5 carries a foil US of zirconium or other heatresponsive getter material. This foil Hi can conveniently be secured in position by a clamping ring I! fastened to the partition i5 by screws l8. To permit free communication between the two ends of the X-ray tube on either side of the partition IS, the latter is provided with at least two apertures i9. These apertures 19 are preferably adjacent the outer edge of the partition l5 and are distributed symmetrically about the axis of the tube.

The foil it may be provided with a central opening before being mounted or it may be apertured by cathode bombardment in precisely-the same manner as hereinbefore described for the foil 9, Fig. l. it

Although only two specific examples have, been described and illustrated, each involving the use of a single foil, it will be aparent that two or more foils may be mounted in parallel relationship by means identical with or equivalent to those described with reference to the drawing. Conseanode, a cathode, and

getter material arranged transversely to the disquently the invention is not to be interpreted as confined to the preferred embodiments or otherwise limited except by the appended claims.

I claim:

1. An electron discharge tube comprising an anode, a cathode, and a foil of heat-responsive getter material arranged transversely to the discharge path between said cathode and anode, said foil having an aperture to permit the passage of the discharge.

2. An electron discharge tube comprising an a foil of heat-responsive charge path between said cathode and anode, said foil being so constituted that an aperture may be formed therein by electron bombardment.

3. In the manufacture of an electron discharge tube the steps of mounting a foil of heat-responsive gettermaterial in the path of the discharge between cathode and anode, degassing said foil,

and subsequently bombarding said foil with electrons and evaporating that part of the foil which lies in the direct line of the discharge.

4. An X-ray tube comprising an evacuated envelope, a cathode, an anode, and a foil of heat responsive getter material arranged within said envelope transversely to the discharge path, said foil having an aperture to permit the passage of the discharge,

5. An X-ray tube comprising an evacuated envelope, a cathode, an anode, a hood extending from said anode towards the cathode so as to surround the path of discharge adjacent the anode, and a foil of heat-responsive getter material carried by said hood and disposed transversely to the path of discharge, said foil having an aperture to permit the passage of the discharge.

6. An X-ray tube comprising an evacuated envelope, a cathode, an anode, a hood extending from said anode towards the cathode so as to surround the path of discharge adjacent the anode, and a foil of v heat-responsive getter material mounted transversely across said hood at a point spaced from the anode surface, said foil having an aperture for the passage of the discharge.

'7. An X-ray tube comprising a cathode, an anode, an evacuated envelope having a metallic portion surrounding the discharge path between said cathode and anode, a wall integral with said metal portion and extending inwardly therefrom towards the discharge path in a plane normal'to said path and between said cathode and anode and having an opening therein greater than the cross-section of the discharge, and a foil of heatresponsive getter material carried by said wall and extending across the opening therein, said foil having an aperture for the passage of the dis- 1 charge.

8. In the manufacture of an X-ray tube the steps of mounting a foil of heat-responsive getter 7 material between anode and cathode in a plane at 

